The invention generally relates to radiological devices that create the image of an object through the use of an X-radiation that irradiates the object. It relates, more particularly, to a method, an X-ray tube and a system for the elimination, from the image of the object, of the image corresponding to a fixed antiscatter grid.
The invention shall be described in its implementation into a mammography apparatus of the type shown schematically in FIG. 1.
This mammography apparatus has an X-radiation source 10 borne by a bracket 11 positioned at the top of a vertical plate 12. This vertical plate 13 has an assembly 13 on which there lies a breast 16 to be examined by means of a horizontal shelf 15 as a support. A pad 17, that is transparent to X-radiation and is vertically movable, is used to compress the breast.
To match the size of the breast, the plate 12 is mounted on a vertical column 9 resting on the ground, and shifts vertically on said column by means of an appropriate mechanical device.
On its upper part and beneath the shelf 15, the assembly 13 has a tunnel in which there is housed a cartridge 18 constituted by a black box enclosing a film 14 sensitive to direct X-radiation or to a photon radiation emitted by a screen (not shown) that receives the X-radiation. It is on this film 14 that there is formed the latent image of the breast after an appropriate exposure time. The development of the film gives a radiographic picture.
In a mammograph, the breast 16 under examination is close to the radiographic film 14, and as a result of scattering by the different points of the breast, the contrast and resolution of the image are diminished. To prevent this phenomenon, a grid 19, known as an antiscatter grid, is interposed between the breast and the radiological film. This grid 19 eliminates all the scattered rays having an angle of incidence on the film 14 that is greater than a certain value.
A grid such as this (FIGS. 2 and 3) has a succession of lead strips 20, lead being a material that absorbs X-radiation, separated by interposed elements 21 made of a material that absorbs little X-radiation, for example, a material such as paper or aluminium. These strips are oriented convergently towards the focal spot of the X-ray tube. Typically, the strips have a thickness d=10 to 30 microns for a pitch of D=100 to 300 microns and a height h of a few millimeters.
Since a grid such as this has strips that are opaque to X-radiation, each of these strips gives an image on the radiological film. Hence, to eliminate this image of the grid, there is a known way of communicating an alternating motion of a few hertz to the grid to make it invisible on the radiological image. This alternating motion is generally obtained by a motor 22, which increases the space occupied by the unit.